The purpose of this project is to assess and compare the psychosocial-physiologic and cognitive effects of long term dependence to both methadone hydrochloride and l-gamma-acetylmethadol in the substitution and maintenance therapy of heroin addicts. More specifically, it is to determine: (1) the rate and degree of tolerance development to the effects of these agents; (2) the residual behavioral and physiologic effects that remain at "maintenance" doses; (3) the occurrence of any untoward psychotoxicity or impaired cognitive function that might result from their use, and (4) how the observed direct effects of the drugs correlate with the social adjustments made.